The invention relates to a glazing comprising a means for detecting a crack in the glazing.
Glazing may be provided with electrical devices such as crack detection elements, heating elements and antennas. Crack detection elements perform a damage detection function and are connected to a crack detection module which generates an alarm.
Heating elements perform defogging and de-icing functions and may comprise a transparent electrically resistive coating connected to a power supply. Antennas perform the function of receiving or transmitting data by electromagnetic waves, for example AM/FM radio, TV and mobile telephones.
It is known to provide an automotive glazing comprising a frangible wire for crack detection positioned around a periphery, for example DE9303273U1 (Trebe). Such a car alarm is sensitive to breakage by an intruder, causing glazing rupture.
It is known to provide a crack detector comprising a glazing having a conductive wire, connected via a terminal to an oscillator. In JP2003085660 (Asahi/Shibata) a glazing breakage is detected as a change in resonant frequency of the oscillator.
It is known to provide an antenna on a glazing connected via connection points to an alarm evaluator for crack detection. In DE19501103 (Flachglas/Paulus) an antenna in the viewing area of an automotive glazing performs two functions: an antenna for an AM/FM amplifier and a crack detection element for an alarm evaluator. The alarm evaluator applies DC voltage or low-frequency single-phase AC voltage to the antenna and measures resistance of the antenna. To prevent undesirable mutual influencing of the two functions of the antenna, connection between the AM/FM amplifier and the antenna is via a capacitor.
Crack detectors may be divided into two categories: “all or nothing” electronics modules, which require substantially complete opening of a circuit, and “threshold” electronics modules, which respond to a change in resistance of a circuit. “All or nothing” electronics modules can be very much simpler, and therefore less expensive. GB2190877 (Glaverbel/Laurent) discloses a laminated glazing comprising unequally stressed plies of glazing material, so that cracks reliably propagate from side to side of the glazing causing rupture, to allow use of an “all or nothing” electronics module.
Cracks in an outer ply of a laminated heated coated windscreen not resulting in rupture may cause a discontinuity in a coating or a busbar. Arcing or non-uniform current distribution around such a discontinuity may cause a hotspot in a part of the glazing and ineffective heating in another part. A hotspot may cause the crack to propagate, affecting the safety function of the windscreen. A hotspot may cause a fire.
It is known to detect a crack by measuring voltage across a coating or resistance of a coating. For example, in U.S. Pat. No. 4,565,919 (Donnelly/Bitter) a coating voltage is monitored and in EP1648200 (Pilkington/Dixon) an electronics module repeatedly determines a is threshold resistance based on a plurality of previous coating resistance values.
It is known to measure resistance of a sensor wire, connected to a top busbar, for example U.S. Pat. No. 4,829,163 (GM/Rausch), U.S. Pat. No. 4,902,875 (PPG/Koontz) and JPH06087407 (Denso/Ito). In EP0430119 (PPG/Koontz) two sensor leads are connected to tabs at predetermined positions on a coating. Disadvantageously, preferred embodiments of these disclosures require five edge connectors.
U.S. Pat. No. 4,939,348 (PPG/Criss) discloses a sensor lead which encircles a periphery of a glazing. A power source voltage for a heating element on the glazing is also applied to the sensor lead, permitting a voltage change to be detected in the event of a crack at any edge portion of the glazing. An electronics module detects a break in the sensor lead and generates a signal. U.S. Pat. No. 4,939,348 requires four edge connectors.
It is an object of the present invention to provide a glazing comprising an alternative means for detecting a crack in the glazing.